At Havasu last November, most were running about 4" with loose tension for very light winds ~ 1 3/8" prebend(I surveyed most of division 3 sailors including Tim Parsons, Mark Zimmer, George Pedrick, Rafi Yalholm and had Tim check my rig)

For heavy winds and seas in Santa Cruz kick off this year after talking with Tim, I set it to be about 1 1/2 to 1 5/8 rake with heavy tension to get about 1 3/8" prebend.I don't have a gauge to determine the tension yet and I am trying to learn which setting is best.Tim even adjusts his diamond wire tension on the water depending upon wind conditions. I know he and Mark Lewis did a series of 2 boat tune ups to play with rake and tension.

Here is what I found on the internet:

Spreader rakeSpreader rake is a method of tuning the fore-aft stiffness of a mast below the hound fitting. Crew weight affects the required amount of spreader rake as it has a significant effect on the amount power developed by the rig.To answer how much is sufficient rake can only be determined by sailing the boat and knowing what to look for.If you have excellent height up wind, but lack boat speed and the boat won’t accelerate in the wind gusts,then you need more rake. This helps the mast bend fore and aft which allows the sail to flatten and theleech to open in the wind gusts. (Also see notes on diamond tension.)

If you are lacking height and "grunt" in light to medium weather, then you have too much spreader rake.

The spreader rake is measured by placing a straight edge or string-line between the diamond wires at thespreaders and measure the distance to the back of the mast.

Settings depend heavily on the weight of the crew, with heavier crews generally carrying less rake than lighter crews.

Diamond tensionThe tension on the diamond wires primarily controls the side bends of your mast. Upwind loose diamonds allow the middle of the mast to bend to leeward and the top of the mast to hook to windward. This tends to cause the boat to heel very easily in wind gusts. Generally speaking, diamond tension is a way of powering or depowering your rig. Wind the tension on to depower and wind them off to power up.